Hydrogen-bond networks: Strengths of different types of hydrogen bonds and an alternative to the low barrier hydrogen-bond proposal

We report quantifying the strengths of different types of hydrogen bonds in hydrogen-bond networks (HBNs) via measurement of the adiabatic electron detachment energy of the conjugate base of a small covalent polyol model compound (i.e., (HOCH₂CH₂CH(OH)CH₂) ₂CHOH) in the gas phase and the pK_a of the corresponding acid in DMSO. The latter result reveals that the hydrogen bonds to the charged center and those that are one solvation shell further away (i.e., primary and secondary) provide 5.3 and 2.5 pK_a units of stabilization per hydrogen bond in DMSO. Computations indicate that these energies increase to 8.4 and 3.9 pK_a units in benzene and that the total stabilizations are 16 (DMSO) and 25 (benzene) pK_a units. Calculations on a larger linear heptaol (i.e., (HOCH₂CH₂CH(OH)CH₂CH(OH)CH ₂)₂CHOH) reveal that the terminal hydroxyl groups each contribute 0.6 pK_a units of stabilization in DMSO and 1.1 pK _a units in benzene. All of these results taken together indicate that the presence of a charged center can provide a powerful energetic driving force for enzyme catalysis and conformational changes such as in protein folding due to multiple hydrogen bonds in a HBN.